The invention relates to an optical system for tracing an information recording medium such as an optical disk for recording and reproduction or recording, reproduction and erasing of information with multiple beams and, more particularly, to an optical system which splits a single light beam into a plurality of light beams.
Recently, various optical heads for recording and reproducing information on and from optical disks have been developed and used in various ways. In such an optical head, a laser beam which is light intensity modulated according to information to be recorded is focused by an objective lens onto a rotating recording surface of the optical disk to cause a continuous change in the state of the recording surface, e.g., form pits or like holes or change an optical characteristic such as the light reference or reflectivity of the disk, whereby information is recorded on the recording surface. Also, when a reproduction laser beam is focused by the objective lens onto the recording surface, it is light intensity modulated by the areas of the continuously changed state. Information is read out as the reproduction light beam is converted by an optical detector into an electric signal.
An optical disk which has a tracking guide for specifying an area, in which information is to be recorded, is well known in the art. An optical head, which records information on an optical disk while confirming that the information is being accurately recorded on the tracking guide, is also know. In such an optical head, in which a reproduction beam is focused on the optical disk not only during reproduction but also during recording, an area that has been traced by a recording beam is also traced by a reproduction light beam. Therefore, it may be impossible to prevent the possible departure of a recording light beam from a predetermined tracking guide. Therefore, it may be impossible to prevent damage that may be caused to an area other than the recording area by the recording light beam. Further, there may be defective areas in the tracking guide, which cannot be detected, so that it is possible to record information in the defective areas.
In order to solve the above problems, optical heads have been proposed in which a prebeam or guide beam is converged onto an optical disk in addition to a recording/reproduction light beam and the area that has been traced by the recording and reproduction light beam, as disclosed in U.S. application Ser. No. 673,764 filed Nov. 21, 1984 and corresponding EPC application No. 84114044,5 filed Nov. 20, 1984. In such optical heads, it is required that a plurality of light beams be incident on the optical disk at slightly different incidence angles. No optical system which can meet such a requirement, however, has yet been proposed.